The invention relates to a high pressure mercury discharge lamp comprising
a quartz glass lamp vessel having a region surrounding a discharge space;
spaced-apart tungsten electrodes defining a discharge path, disposed in the lamp vessel, and connected to current conductors which extend from the lamp vessel to the exterior;
a filling of at least 0.2 mg Hg/mm.sup.3, 10.sup.6 -10.sup.-4 .mu.mol/ Hal/mm.sup.3, wherein Hal is selected from Cl, Br and I and rare gas in the discharge space.
Such a lamp is known from EP 0 338 637-A2.
The known lamp has the advantage that, owing to its high operating pressure of at least 200 bar, its radiation contains a substantial amount of continuous radiation in the visible portion of the spectrum. The lamp has a long life, a high lumen maintenance and a small variation of its color point during its life.
The lamp known from said EP Application has an elongate, narrow, cylindrical or elliptical lamp vessel and consumes a low power of no more than 50 W. For many purposes, such as e.g. image projection, the luminous flux of the known lamp is too small. The lamp is, however, already highly loaded by more than 1 W/mm.sup.2.
Investigations revealed that in order to obtain the high operating pressure, it is necessary to achieve a temperature of at least about 1160 K at any spot inside the lamp vessel. On the other hand, however, no spot of the wall of the discharge space is allowed to have a temperature of more than about 1390 K. Higher temperatures would induce crystallization of the quartz glass, which would bring about the destruction of the lamp vessel. Thus, the range of temperatures between the minimum temperature required and the maximum temperature permitted is very narrow.
This narrow range prevents the known lamp from being more highly loaded in order to consume a higher power. Also, it appears to be impossible to obtain a higher power consumption, while maintaining a long useful life, by enlarging the dimensions of the lamp vessel by normal up-scaling methods. In doing so, convection currents inside the discharge space would increase. This would have the effect that wall portions above the discharge would get an increased thermal load, whereas portions below the discharge would be loaded at too low a level.
Nevertheless, there is a strongly felt need for a lamp of very high luminosity, comparatively stable, comparatively high luminous efficacy, comparatively stable color point and long life, and a higher luminous flux than the lamp of the opening paragraph, e.g. for LCD projection TV. Metal halide lamps, for instance, fail in this respect, since the relatively large amounts of halogen present as halides cause corrosion of the electrodes. This results in color shifts, power changes, wall blackening and a reduced light output.